This invention relates generally to the control of direction and speed of movement of motor driven vehicles, particularly those using hydrostatic transmission mechanisms having a control member operative to place the transmission mechanism selectively in a neutral condition and forward and reverse drive conditions. Such transmission mechanisms are used in lawn and garden tractors, these often traversing uneven ground surfaces. Many such tractors are not generally spring mounted, and possess rather rough riding qualities, only the operator's seat being spring mounted or otherwise cushioned. In such vehicles, a foot pedal or other manually operated devices, such as levers, are used to operate the transmission control member to provide forward and rearward movement and to control the speed of the vehicle during such movements. When the vehicle runs over rough ground, the operator's foot, in the case of a pedal control, is often jarred or jostled to the point of causing the speed of the vehicle to be altered, the jarring movement being difficult to control once it is initiated. Under such conditions, the operator of the vehicle is placed under considerable strain in endeavoring to maintain the vehicle at a constant operating speed, resulting in early and undue fatigue to the operator. The mechanism of this invention effectively prevents involuntary movement to be imparted to the transmission control member, except when the operator's foot is removed from the pedal or, as the case may be, the hand from a control lever; and enables the vehicle to be driven at a substantially constant speed over rough and uneven ground. The transmission mechanism, having means biasing the same toward its neutral condition, assumes this condition when the operator's foot is removed from the pedal or the hand from the control lever, as the case may be.